Hydroxy ether compounds have important uses as solvents, coalescents, surfactants, wetting agents, emulsifying agent, and are widely used in consumer good and industrial applications such as cleaning supplies and coating materials.
In the past, the manufacture of hydroxy ether compounds, that is compounds of the formula ROR2OH, where R and R2 designate substituted or unsubstituted or branched or unbranched alkyl groups, was typically accomplished by alkoxylation reactions involving alcohols and alkyl epoxides. This conventional process has proven to be somewhat inefficient, in that it produces various undesirable byproducts along with the ether alcohols. Further, it is difficult to control the number of alkylene oxide units added to the alcohol during production, resulting in undesirable molecular weight distributions. These reactions typically exhibit poor selectivity to the more desired monoether product producing a mixture of compounds of the formula RO(R2O)nR2OH where n is an integer from 2-8.
This selectivity issue can be addressed through hydrogenolysis of the product from acetalization reactions of aldehydes and diols, namely hydrogenolysis of cyclic acetals and cyclic ketals in the presence of a noble metal catalyst. In addition, safety concerns of dangerous alkyl epoxides can be mitigated by eliminating the need to directly handle or transport these materials. Another benefit of this technology lies in its ability to produce products from starting materials derived from renewable resources in the manufacture of 1,3-propanediol and glycerin without use of the Williamson method of synthesizing ethers which leads to low yields, high amounts of waste salt products, and requires handling of halogenated alkylating agents.
The hydrogenolysis of cyclic acetals and ketals to hydroxy ether compounds, while more selective than traditional alkoxylation, does have non-selective by-products. For example, a common by-product is formed by the reaction of one mole of acetal with one mole of hydroxy ether product, followed by hydrogenolysis to yield 1,2-dialkoxy-alkanes. This particular side reaction is a two-mole loss of starting material. Successful hydrogenolysis processes desirably reduce this side reaction as much as possible. Other detected side products include esters from internal rearrangement and ester alcohols from trans-esterification reactions.
For example, U.S. Pat. No. 4,484,009 describes the preparation of glycol monoethers by hydrogenolysis of a 2-alkyl-1,3-dioxolane with hydrogen in the presence of a noble metal catalyst such as palladium, an acid of phosphorus or ester of phosphorus as a co-catalyst, hydroquinone, and in a solvent system such as ethylene glycol.
In these processes, the use of phosphoric and other similar phosphorous-containing acid co-catalysts or halides of Group III metals and hydroquinone type additives as promoters is common. The liquid phase hydrogenolysis of cyclic acetals to ether alcohols (hydroxy ether compounds) using Ni catalyst as an active metal has met with limited success in hydrogenolysis of alkyl ethers. Such processes require handling of extra starting materials and removal of more undesirable ingredients to purify the end product. It is desirable to conduct the reaction in the absence of a co-catalyst which has difficulty in separation and lead to potential corrosion, while obtaining good selectivity to the desired hydroxy ether compound.
U.S. Pat. No. 4,479,017 describes a process for hydrogenolysis of cyclic acetals in the presence of a palladium over a carbon support and a solvent, if used, can be selected from among alcohols, ethers, and hydrocarbons.
One-pot reaction systems have also been reported, that is, reacting an aldehyde and a polyol with hydrogen in the presence of a noble metal catalyst directly to the desired ether alcohol. For example, U.S. Pat. No. 5,446,210 describes a process for the production of hydroxy ether compounds in a one pot system by reacting a polyol with an aldehyde and hydrogen in the presence of a noble metal catalyst where the molar ratio of polyol to carbonyl compound of 5:1 to 1:5 is described, but with these molar ratios, the yield was low in the range of 35 to 50% even including the bis-types of by-products with low selectivity to the mono-ether products.
US Publication No 2010/0048940 also describes a one-pot system in which a polyol and a carbonyl compound and hydrogen are reacted together in the presence of a hydrogenolysis catalyst to provide the polyol ether in which the molar ratio of polyol to carbonyl compound was at least 5:1 to improve selectivity and yield. In the one-pot system, the selectivity to the 2-butoxy ethanol from the reaction of ethylene glycol, buteraldehyde, and hydrogen reported in this publication did not exceed 90%, although reactions with diethylene glycol and tetraethylene glycol were reported to have yields of 94.5% and 91.2%, respectively. Examples of a two-stage process in which the acetal compound was first synthesized and then subjected to hydrogenolysis were reported as having even lower yields than those examples given for a single step synthesis.
In many of the examples in the literature, the processes were conducted on a batch basis. On a commercial scale unit, the process must run on a continuous basis and operated economically which requires, among other considerations, the use of a catalyst having good activity, in a process having good selectivity toward the desired hydroxy ether compound. There is a continued need to be able to carry out the efficient catalytic hydrogenolysis of cyclic compounds to make desired hydroxy ether products in high selectivity on a continuous basis, desirably without the need for co-catalysts.